Technical Field
Embodiments of the subject matter disclosed herein generally relate to an apparatus and method for vibration mitigation through sequential impedance optimization.
Discussion of the Background
An electro-mechanical cable may be a cable, such as, for example, a marine-seismic cable, including sensor components, data-transmission cables, power-transmission cables and strength enhancing and buoyancy enhancing components arranged in a single cable. A marine-seismic cable may be an electro-mechanical cable used for gathering data on the nature and composition of the earth below a body of water using seismic imaging techniques. The marine-seismic cable, or seismic streamer, may be designed to reduce hydrodynamic induced flow noise. For example, the marine seismic cable may be cylindrical.
FIG. 1 depicts an exemplary marine-seismic cable system in use. A marine vessel 101, including a lead-in towing assembly 102, may deploy and tow electro-mechanical cables 103, on or below the surface of the water. The electro-mechanical cables 103 may be of any suitable length, and may be made up of shorter connected sections 106 of electro-mechanical cabling which may also be of any suitable length. For example, an electro-mechanical cable 103 may be kilometers in length, with each section 106 being, for example, 75 meters to 200 meters in length. Sections 106 may be detachable from each other. The electro-mechanical cable 103 may be, for example, a marine-seismic cable or seismic streamer. Seismic-imaging systems may make use of more than one electro-mechanical cable 103. The electro-mechanical cables 103 may be deployed as a single section 106, a linear series of sections 106, or as a horizontal offset series of sections 106, producing a sensor field.
Sections 106 of the electro-mechanical cables 103 may include various sensor components 104. Sensor components 104 may be, for example, hydrophones, geophones, accelerometers, electro-magnetic sensors, optical sensors, gravity sensors, or a combination thereof and may be distributed at regular intervals along the electro-mechanical cables 103. An outer jacket around the electro-mechanical cable 103 may be, for example, a polyurethane jacket, and may be smooth in order minimize noise in the sensor components 104. A buoyant material may be contained in the electro-mechanical cable 103, and may help keep the electro-mechanical cable 103 level on top of or under the water.
A seismic source 108 may be used to produce a shockwave, using any suitable manner of generating acoustic energy. The seismic source may include one or more air guns or vibratory elements. When conducting an acoustic survey, the shockwave may be reflected by the geologic features of the sea floor and picked up by the sensor components 104. Vibrations emanating from the head of the streamer field (or streamer spread) 110, near the marine vessel 101, may contaminate the seismic signals measured by the sensor components 104. Radial vibration isolation modules 109 may be placed between the towing assembly 102 and the electro-mechanical cables 103, at the head of the streamer field 110, in order to mitigate the transmission of vibration noise. There are several types of radial vibration isolation modules 109 that may be used in electro-mechanical cables 103 at the head of the streamer field 110. Each of the available types of radial vibration isolation modules 109 may include a single stretch section with vibration attenuation that occurs due to a complex spring rate of the radial vibration isolation module 109. Radial vibration isolation modules 109 may be tailored to attenuate vibration over the frequency bandwidth of 2 to 250 Hz, which may be common in seismic acquisition.
The electro-mechanical cables 103 may each include one or more positioning devices, also known as birds 107. Birds 107 may include control surfaces that may be used to position the electro-mechanical cables 103. For example, the birds 107 may be used to maintain the electro-mechanical cables 103, to which they are attached, in a known and controllable position relative to other electro-mechanical cables 103. The birds 107 are capable of moving the electro-mechanical cables 103. The electro-mechanical cables may also include attached recovery nodes, which may be devices clamped to the outsides of the electro-mechanical cables 103 that may include sensors (pressure sensors) that monitor for when the electro-mechanical cables 103 pass a given depth. The recovery nodes may include an inflatable portion that may inflate if an electro-mechanical cable 103 has sunk too far into the water, causing the electro-mechanical cable 103 to float back to the surface where it can be retrieved.
Because the electro-mechanical cables 103 are deployed in a viscous fluid, for example, water, the electro-mechanical cables 103 are subject to energy sources from both man-made sources, such as energy transmitted through the tow assembly 102, or energy from the propulsion system of the marine vessel 101, and natural sources, such as wave motion and weather. The energy from these sources may diminish the quality of the seismic data recorded by the sensor components 104, as they may interfere with the signal from the acoustic energy reflected off the sea floor. Thus, the signal measured by the sensor components 104 of an electro-mechanical cable 103 may be divided into two parts, the “signal” pertaining to the geophysical structure of the sea floor, and “noise,” which may be picked up from other man-made or natural sources. The signal is desired while the noise contaminates the signal.
Existing noise suppression hardware, such as the radial vibration isolation modules 109, which are concentrated at the head of the streamer field 110, between the towing assembly 102 and the electro-mechanical cables 103, may not provide a sufficient level of noise abatement because devices that operate over such a wide frequency bandwidth, e.g., 2-250 Hz, are typically a result of compromise, sacrificing performance in one frequency region to handle another. The “noise” experienced by electro-mechanical cables 103 may be both spatially dependent, varying depending on the position within the streamer field 110, and frequency dependent. Thus, there is a need for an apparatus and method for vibration mitigation that overcomes the problems mentioned above.